Temporal coding expands the bandwidth of GPCR-mediated neuromodulation

The advent of modern transcriptomic approaches has revealed a breadth of neuromodulatory G protein-coupled receptor (GPCR) co-expression far exceeding what was previously thought. This raises a fundamental question about neuromodulatory system architecture because there is a much smaller number of available G protein transducers. Does this impose an information bottleneck, or can a single transducer pathway distinguish the effects of different neuromodulators? We addressed this question by focusing on four GPCRs that are natively co-expressed in essentially all hippocampal pyramidal neurons and that signal through the canonical Gs-coupled cyclic AMP (cAMP) cascade. We show that each GPCR produces a similar cAMP elevation upon acute activation but the effects observed at downstream steps diverge, to the point that the GPCRs differ dramatically in their ability to drive a transcriptional response. We also show that they differ in their ability to maintain functional responsiveness, with neuropeptide receptors remaining responsive for hours but monoamine receptors rapidly losing responsiveness. We conclude that the effective chemical bandwidth of cellular neuromodulation by GPCRs is not limited by the number of available transducers, and that there exists additional temporal coding which enables individual neurons to distinguish a richer neuromodulatory input.